1. Field of the Invention
The present invention relates to a process for improving the printability of paper and paper products when printing with the aid of the ink-jet printing method by treating the paper or the paper products with aqueous solutions of cationic polymers.
2. Discussion of the Background
Printing of papers, paper-like materials or textiles using digital printing methods is becoming increasingly important in the printing industry. These digital printing methods also include the ink-jet printing method.
In the classical printing methods, a printing plate treated with ink is pressed onto the paper. The printing inks are in most cases not dissolved in water but in an organic solvent, such as toluene. In contrast, in the ink-jet printing method, drops of an ink generally dissolved in water are sprayed onto the recording material from a nozzle according to the contours present on the copy. The user's requirements and the requirements of the ink-jet printing method with respect to the recording material, e.g. paper, are therefore of a very different kind compared with the classical printing methods. However, as in all other printing methods, the requirements with respect to the printed image and hence with respect to the paper used also vary very considerably in terms of quality, depending on the purpose for which the image is intended. For images which are to have the quality of a photograph, an appropriately high-quality paper must be used. For very simple messages and designs on paper which are not to be stored, papers of a quality which permit reading or recognition of the message or of the image are sufficient. Between these two extremes, there is a wide range of papers which have to meet the respective requirements for ink-jet printing. High-quality papers suitable for this printing method are provided, for example, with a coating of a water-absorptive pigment, a hydrophilic binder and a cationic polymer (cf. G. Morea-Swift, H. Jones, THE USE OF SYNTHETIC SILICAS IN COATED MEDIA FOR INK-JET PRINTING, in 2000 TAPPI Coating Conference and Trade Fair Proceedings, 317-328). For fairly undemanding ink-jet prints, natural papers having a simple starch coating are frequently used.
In contrast to most other printing methods, the ink-jet printing method has the basic disadvantage that the printouts are water-sensitive owing to the water solubility or the water dispersibility of the inks used. When the printed image comes into contact with water, this leads to running of the inks into one another and into the paper, both in the plane of the paper and perpendicular to the plane of the paper. In the most unfavorable case, a script is then no longer legible, an image is blurred and the inks strike through to the back of the paper. Of course, the water resistance of the printout is not of major importance for all users, but, even in the case of users whose images usually do not come into contact with water, it is very annoying if water accidentally drips onto the image and smudges it and if the ink which has run out soils the tablecloth, or the image shows the imprints of moist fingers. For papers which are printed on with the aid of the ink-jet printing method and which are exposed to rain, for example placard papers or packaging papers, or which may become moist through condensation or filling liquids, e.g. bottle labels, water resistance of the ink-jet printout is indispensable. If, for example, the bar code on a packaging or on a label is no longer crisp owing to the action of moisture and is not read or is incorrectly read, the economic damage may be very great. High-quality expensive papers for ink-jet printing, as used, for example, for photography, art printing, etc., give water-resistant printed images. They are produced by coating base paper with an ink consisting of a water-absorptive pigment, preferably silica, a water-soluble binder, preferably polyvinyl alcohol, if required further water-soluble binders, and cationic organic polymers (cf. G. Morea-Swift, H. Jones, THE USE OF SYNTHETIC SILICAS IN COATED MEDIA FOR INK-JET PRINTING, above).
WO-A-03/021041 discloses that the whiteness of recording materials can be increased if mixtures which comprise an optical brightener, a cationic polymer and a solvent are applied thereon. The recording materials thus obtainable can be printed on by the ink-jet method. They give substantially better color reproduction and crispness of contours than conventional papers.
For the large amount of relatively simple papers, e.g. office papers, such special coatings are too complicated, too expensive and frequently also unsuitable for conventional processing of the papers, for example for printing by the offset method, for copying, for simple writing on using ink or for drawing using a pencil and erasing of the drawing. The printing of daily newspapers from the Internet on an ink-jet printer is becoming more and more fashionable. For the abovementioned reasons, a simple paper which gives a water-resistant ink-jet printout is required for this purpose.